The invention relates to the field of wind turbine generators and specifically to wind turbines that rotate about a vertical axis.
Vertical axis wind turbines have been known for many years. The most common design for a vertical axis turbine is the Darrius turbine that uses curved blades in a troposkien shape. Other vertical axis turbines use straight blades that are attached to a vertical shaft with one or more blade support arms.
Modern vertical axis turbines use airfoils that provide lift rather than using aerodynamic drag to provide motive force to the rotor. The use of lift-producing airfoils increases the aerodynamic efficiency of the rotor greatly compared to a drag-type device. However, even with lift-producing airfoils, conventional vertical axis turbines suffer several disadvantages compared with horizontal axis turbines. The peak aerodynamic efficiency that most vertical axis wind turbines achieve is approximately 25-30%. Also, vertical axis wind turbines are not inherently self-starting and require the use of a starting motor to allow them to begin turning. Several improvements to the basic vertical axis wind turbine design have attempted to solve the inherent problems.
U.S. Pat. No. 4,115,027, the Specification of which is incorporated herein by reference, discloses a vertical axis, lift type windmill. Vertical airfoils that provide aerodynamic lift are mounted with struts around a central shaft to form a rotor.
U.S. Pat. Nos. 5,057,696 and 5,332,925, the Specifications of which are incorporated herein by reference, disclosed various improvements to the windmill of U.S. Pat. No. 4,115,027. The improvements included a new braking system, the use of thick airfoils, a drive belt transmission, two speed operation, and rotatable stators that improve efficiency and limit structural loads during high winds.
The wind turbines in all of the above-referenced patents utilize stationary fairings on the outside of the rotor to direct wind flow through the rotor and increase efficiency. While this approach has been found to significantly improve the performance of the wind turbine (aerodynamic efficiency has been measured as high as 52%), it also results in additional structure that must be supported and it adds planform area which increases the wind loading on the structure during storm conditions. It would be desirable to achieve similar performance improvements without the need for structural elements such as stationary fairings.
The wind turbines of the above referenced patents also utilize a mechanical braking system that has been found to be very reliable but which also requires a manual reset after activation. This can lead to high turbine downtime and low availability if an operator is not present on site full time. It would be desirable to incorporate a braking system that engages automatically when a fault occurs in the turbine system and that resets automatically and returns the turbine to service after a fault condition has been cleared.
It would be desirable to provide a vertical axis wind turbine that achieves high aerodynamic efficiency while requiring minimal support structure. It would also be desirable to provide a vertical axis wind turbine that is suitable for use underneath an existing row of horizontal axis wind turbines in a xe2x80x9cbush-treexe2x80x9d configuration in order to maximize energy production on a parcel of land. It would further be desirable to provide a vertical axis wind turbine that incorporates a robust and reliable automatic aerodynamic and mechanical braking system that self-resets after a fault is cleared. Further, it would be desirable to minimize maintenance frequency and difficulty by providing easy access to parts requiring more frequent attention such as the gearbox and generator. It would be desirable to structurally simplify the turbine even further by employing a guyed shaft structure or a combination of guyed and externally supported frame rather than employing an external support frame. An externally guyed structure would minimize the number of parts required. These structures can also provide a cleaner aerodynamic flow field to enhance the turbines"" vortex effect. It would be further desirable to enhance the turbines"" vortex effect and improve self-starting capabilities by employing a high solidity rotor.
The present invention provides a wind turbine with increased aerodynamic efficiency through vortex interaction between two adjacent wind turbines and by employing high solidity rotors. The vortex interaction is the result of close placement of adjacent turbines as well as their angular orientation relative to the direction of the prevailing energy winds. The adjacent turbines must also rotate in opposite directions in order to achieve the coupled vortex interaction.
The guyed shaft structure can be placed in close proximity in a row configuration by employing either a three or four cable support point configuration in a staggered configuration that provides cable-to-cable and cable-to-rotor clearance. The wind turbines can be arranged in a long row of coupled wind turbines with aerodynamic enhancements occurring throughout the row of turbines. The row of turbines should be oriented perpendicular to the prevailing energy wind direction. This orientation of turbines is particularly well suited to geographical areas with a strong prevailing wind direction and little directional variability.
The row of vortex coupled turbines can be located underneath a row of horizontal axis turbines. This xe2x80x9cbush-treexe2x80x9d configuration maximizes the energy capture that can be derived from a parcel of land. It is also possible that the aerodynamic performance of the horizontal axis turbines could be improved because of the presence of the vertical axis turbines beneath them It is possible that the row of vertical axis turbines could provide a vertical mixing effect bringing higher energy flow into the horizontal axis wind turbines"" flow field.
The turbine uses a pneumatic braking system that automatically releases and allows the turbine to resume operation after a fault condition has been cleared. The pneumatic brake is biased with a weight so that it is normally applied via the weight and released when a pneumatic cylinder is pressurized to lift the brake and the weight. A solenoid valve that is normally closed controls the pressure to the pneumatic cylinder. The valve is electrically activated. When electrical power is off, the valve opens to release pressure to the cylinder. When power returns, the valve closes and a compressor pressurizes the cylinder to lift the weight and release the brake. This assures that the brake will be applied to stop the turbine in the event of a loss of electrical power and that it will be released when power is restored. A toggle switch is provided to open the solenoid valve if power is not lost but if, for some reason, the generator cannot function.
A linkage couples the mechanical brake with a system to adjust the blade pitch. When the brakes are applied, the blades pitch 45 degrees to act as a drag brake. In this way, the turbine has redundant mechanical and aerodynamic braking for greater reliability.
Turbine parts that experience high wear and that require maintenance are located at ground level. The guyed shaft of the turbine is supported by a pair of bearings located at bottom of the shaft. The two bearings are vertically separated by approximately three feet. The uppermost of the two bearings is held in a fixed position and supports the static weight of the main shaft only. The lowermost bearing is free to slide horizontally allowing the guyed shaft to teeter. The lower bearing supports the weight of the blades and the vertical forces from aerodynamic drag on the rotor. The load on the upper bearing is low enough so that the bearing lasts the life of the turbine. The lower bearing is located where it can be easily removed for cartridge replacement. The lower bearing support is free to move horizontally by employing a simple frame of ball bearings.
In the preferred embodiment of the present invention, the turbine includes three sets of blades in three stacked modules. Only the blades in the lowermost module are pitched during braking operations. The bottom set of blades can easily be reached from ground level for maintenance access. It is expected that the pitchable set of blades will require higher maintenance than the other two fixed-pitch sets of blades due to pitching movement during braking. The wear level on the blades is not expected to be high because braking should be a relatively infrequent occurrence. The two upper modules employ blades that are attached with a yoke type attachment with a durable pin connection that restrains the blades from pitching motion and minimizes wear.
The bearing atop the guyed shaft is enclosed within a cable attachment housing and isolated from environmental exposure. This is expected to reduce greasing requirements and minimize wear.